What are atoms with the same atomic number but different atomic masses? | Reference.com What are atoms with the same atomic number but different atomic masses? A: Quick Answer Atoms that have the same atomic number but different atomic masses are called isotopes. The difference in mass arises due to the atoms containing a different number of neutrons for the same number of protons. Full Answer The identity of an element is determined by the number of protons each of its nuclei contains. Protons are positively charged and held together in a small space. The electrostatic force of repulsion between the like charges is very high, but they are able to remain stable due to the presence of neutrons. Neutrons are neutral subatomic particles found within the nucleus of an atom. The protons and neutrons are the heaviest subatomic particles and constitute the entire atomic mass of an element. When two atoms have the same number of protons, they are said to be from the same element. However, if they differ in atomic mass, it means that the number of neutrons in the nuclei is different for the two atoms. Since the number of neutrons does not change the electron configuration of the atoms, the two atoms have the same chemical properties.

However, they have different physical properties, such as boiling point, melting point and density, because their atomic masses are different. To account for the similarities in chemical properties but the difference in physical properties, the two atoms of the same element are called isotopes of that element. For example, hydrogen has two other isotopes, deuterium and tritium, caused by the addition of extra neutrons in the nucleus without a change in the number of protons or electrons. Nuclear Notation Nuclear Notation Standard nuclear notation shows the chemical symbol, the mass number and the atomic number of the isotope . Example: the isotopes of carbon. The element is determined by the atomic number 6. Carbon-12 is the common isotope, with carbon-13 as another stable isotope which makes up about 1%. Carbon 14 is radioactive and the basis for carbon dating . The mass of an element that is numerically equal to the atomic mass A in grams is called a mole and will contain Avogadro's number NA of nuclei. If the density ρ of the material is known, then the number of nuclei per unit volume n can be calculated from n =&#961NA/A.

This is useful in calculating the cross section for nuclear scattering . Go Back Isotopes The different isotopes of a given element have the same atomic number but different mass numbers since they have different numbers of neutrons. The chemical properties of the different isotopes of an element are identical, but they will often have great differences in nuclear stability. For stable isotopes of light elements, the number of neutrons will be almost equal to the number of protons, but a growing neutron excess is characteristic of stable heavy elements. The element tin (Sn) has the most stable isotopes with 10, the average being about 2.6 stable isotopes per element. Information about the isotopes of each element and their abundances can be found by going to the periodic table and choosing an element. Then take the link to nuclear data. Isotopes are (almost) Chemically Identical It is significant to note that the three isotopes of hydrogen change in mass by a factor of three, but their chemical properties are virtually identical. A tiny difference in the spectral frequencies of hydrogen and deuterium comes from an essentially mechanical source, the slight change in the " reduced mass " associated with the orbiting electron.

But for practical purposes the chemical behavior of the isotopes of any element are identical. The dominant contributer to the interactions between atoms and their environment is the electromagnetic force . It should not be surprising that an extra neutron or two in the nucleus has almost no effect on that interaction with the world. Examination of a scale model of the atom makes it evident that the nucleus is extremely tiny compared ot the size of the atom. The nuclear radius of carbon-12 is 2.7 x 10-15 m while the size of the atom from the periodic table is about 0.9 x 10-10 m, about 33,000 times larger! Go Back Nuclear Forces Within the incredibly small nuclear size , the two strongest forces in nature are pitted against each other. When the balance is broken, the resultant radioactivity yields particles of enormous energy. Click on any of the text for more detail The electron in a hydrogen atom is attracted to the proton nucleus with a force so strong that gravity and all other forces are negligible by comparison. But two protons touching each other would feel a repulsive force over 100 million times stronger!! So how can such protons stay in such close proximity?

This may give you some feeling for the enormity of the nuclear strong force which holds the nuclei together. Go Back Nuclear Size The size of the nucleus compared to the size of the atom in which it resides is so small that it has invited a number of interesting comparisons. For example, the space inside an atom can be compared to the space in the solar system in a scale model . Scaling the gold nucleus suggests that the atomic radius is some 18,000 times the size of the nucleus. This great disparity in size was first discovered by Rutherford scattering of alpha particles off a thin gold foil. The extremity of this space comparison is highlighted by the fact that an atom with equal numbers of neutrons and protons, the nucleus comprises about 99.97% of the mass of the atom! Experimental evidence suggests that nuclear matter is almost uniform density , so that the size of a nucleus can be estimated from its mass number. Atomic Number and Mass Number ATOMIC NUMBER AND MASS NUMBERS After reading this section you will be able to do the following: Define and determine the atomic number of an atom. Define and determine the mass number of an atom. What is an atom's atomic number?

The number of protons in the nucleus of an atom determines an element's atomic number. In other words, each element has a unique number that identifies how many protons are in one atom of that element. For example, all hydrogen atoms, and only hydrogen atoms, contain one proton and have an atomic number of 1. All carbon atoms, and only carbon atoms, contain six protons and have an atomic number of 6. Oxygen atoms contain 8 protons and have an atomic number of 8. The atomic number of an element never changes, meaning that the number of protons in the nucleus of every atom in an element is always the same. What is an atom's mass number? All atoms have a mass number which is derived as follows. Review: An element's or isotope's atomic number tells how many protons are in its atoms. An element's or isotope's mass number tells how many protons and neutrons in its atoms. CHEMISTRY I: ATOMS AND MOLECULES CHEMISTRY I: ATOMS AND MOLECULES Table of Contents Atoms | Back to Top Most of the Universe consists of matter and energy . Energy is the capacity to do work.

Matter has mass and occupies space. All matter is composed of basic elements that cannot be broken down to substances with different chemical or physical properties. Elements are substances consisting of one type of atom , for example Carbon atoms make up diamond, and also graphite. Pure (24K) gold is composed of only one type of atom, gold atoms. Atoms are the smallest particle into which an element can be divided. The ancient Greek philosophers developed the concept of the atom, although they considered it the fundamental particle that could not be broken down. Since the work of Enrico Fermi and his colleagues, we now know that the atom is divisible, often releasing tremendous energies as in nuclear explosions or (in a controlled fashion in) thermonuclear power plants. Subatomic particles were discovered during the 1800s. For our purposes we will concentrate only on three of them, summarized in Table 1. The proton is located in the center (or nucleus ) of an atom, each atom has at least one proton. Protons have a charge of +1, and a mass of approximately 1 atomic mass unit (amu). Elements differ from each other in the number of protons they have, e.g.

Hydrogen has 1 proton; Helium has 2. The neutron also is located in the atomic nucleus (except in Hydrogen). The neutron has no charge, and a mass of slightly over 1 amu. Some scientists propose the neutron is made up of a proton and electron-like particle. The electron is a very small particle located outside the nucleus. Because they move at speeds near the speed of light the precise location of electrons is hard to pin down. Electrons occupy orbitals, or areas where they have a high statistical probability of occurring. The charge on an electron is -1. Its mass is negligible (approximately 1800 electrons are needed to equal the mass of one proton). Table 1. Subatomic particles of use in biology. Name electron orbital 9.1095 X 10-31 kg The atomic number is the number of protons an atom has. It is characteristic and unique for each element. The atomic mass (also referred to as the atomic weight) is the number of protons and neutrons in an atom. Atoms of an element that have differing numbers of neutrons (but a constant atomic number) are termed isotopes .

Isotopes, shown in Figure 1 and Figure 2, can be used to determine the diet of ancient peoples by determining proportions of isotopes in mummified or fossilized human tissues. Biochemical pathways can be deciphered by using isotopic tracers. The age of fossils and artifacts can be determined by using radioactive isotopes, either directly on the fossil (if it is young enough) or on the rocks that surround the fossil (for older fossils like dinosaurs). Isotopes are also the source of radiation used in medical diagnostic and treatment procedures. Figure 1. Note that each of these isotopes of hydrogen has only one proton. Isotopes differ from each other in the number of neutrons, not in the number of protons. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Some isotopes are radioisotopes, which spontaneously decay, releasing radioactivity . Other isotopes are stable. Examples of radioisotopes are Carbon-14 (symbol 14C), and deuterium (also known as Hydrogen-2; 2H).

Stable isotopes are 12C and 1H. Figure 2. Carbon has three isotopes, of which carbon-12 and carbon-14 are the most well known. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The Periodic Table of the Elements, a version of which is shown in Figure 3, provides a great deal of information about various elements. An on-line Periodic Table is available by clicking here , Figure 3. The Periodic Table of the Elements. Each Roman numeraled column on the label (at least the ones ending in A) tells us how many electrons are in the outer shell of the atom. Each numbered row on the table tells us how many electron shells an atom has. Thus, Hydrogen, in column IA, row 1 has one electron in one shell. Phosphorous in column VA, row 3 has 5 electrons in its outer shell, and has three shells in total. Image from James K. Hardy's chemistry site at the University of Akron.

Electrons and energy | Back to Top Electrons, because they move so fast (approximately at the speed of light), seem to straddle the fence separating energy from matter. Albert Einstein developed his famous E=mc2 equation relating matter and energy over a century ago. Because of his (and others) work, we think of electrons both as particles of matter (having mass is a property of matter) and as units (or quanta) of energy. When subjected to energy, electrons will acquire some of that energy, as shown in Figure 4. Figure 4. Excitation of an electron by energy, causing the electron to "jump" to another electron (energy) level known as the excited state. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. An orbital is also an area of space in which an electron will be found 90% of the time. Orbitals have a variety of shapes. Each orbital has a characteristic energy state and a characteristic shape. The s orbital is spherical. Since each orbital can hold a maximum of two electrons, atomic numbers above 2 must fill the other orbitals.

The px, py, and pz orbitals are dumbbell shaped, along the x, y, and z axes respectively. These orbital shapes are shown in Figure 5. Energy levels (also referred to as electron shells) are located a certain "distance" from the nucleus. The major energy levels into which electrons fit, are (from the nucleus outward) K, L, M, and N. Sometimes these are numbered, with electron configurations being: 1s22s22p1, (where the first shell K is indicated with the number 1, the second shell L with the number 2, etc.). This nomenclature tells us that for the atom mentioned in this paragraph, the first energy level (shell) has two electrons in its s orbital (the only orbital it can have), and second energy level has a maximum of two electrons in its s orbital, plus one electron in its p orbital. Figure 5. Geometry of orbitals. S-orbitals are spherical, p-orbitals are shaped like a dumbbell or figure 8. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission.

Chemical Bonding | Back to Top During the nineteenth century, chemists arranged the then-known elements according to chemical bonding, recognizing that one group (the furthermost right column on the Periodic Table, referred to as the Inert Gases or Noble Gases) tended to occur in elemental form (in other words, not in a molecule with other elements). It was later determined that this group had outer electron shells containing two (as in the case of Helium) or eight (Neon, Xenon, Radon, Krypton, etc.) electrons. As a general rule, for the atoms we are likely to encounter in biological systems, atoms tend to gain or lose their outer electrons to achieve a Noble Gas outer electron shell configuration of two or eight electrons. The number of electrons that are gained or lost is characteristic for each element, and ultimately determines the number and types of chemical bonds atoms of that element can form. Atomic diagrams for several atoms are shown in Figure 6. Figure 6. Atomic diagrams illustrating the filling of the outer electron shells. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission.

Ionic bonds are formed when atoms become ions by gaining or losing electrons. Chlorine is in a group of elements having seven electrons in their outer shells (see Figure 6). Members of this group tend to gain one electron, acquiring a charge of -1. Sodium is in another group with elements having one electron in their outer shells. Members of this group tend to lose that outer electron, acquiring a charge of +1. Oppositely charged ions are attracted to each other, thus Cl- (the symbolic representation of the chloride ion) and Na+ (the symbol for the sodium ion, using the Greek word natrium) form an ionic bond, becoming the molecule sodium chloride, shown in Figure 7. Ionic bonds generally form between elements in Group I (having one electron in their outer shell) and Group VIIa (having seven electrons in their outer shell). Such bonds are relatively weak, and tend to disassociate in water, producing solutions that have both Na and Cl ions. Figure 7. TOP: Formation of a crystal of sodium chloride. Each positively charged sodium ion is surropunded by six negatively charged chloride ions; likewise each negatively charged chloride ion is surrounded by six positively charged sodium ions. The overall effect is electrical neutrality.

Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. BOTTOM: Table Salt Crystal (SEM x625). This image is copyright Dennis Kunkel at , used with permission. Covalent bonds form when atoms share electrons. Since electrons move very fast they can be shared, effectively filling or emptying the outer shells of the atoms involved in the bond. Such bonds are referred to as electron-sharing bonds. An analogy can be made to child custody: the children are like electrons, and tend to spend some time with one parent and the rest of their time with the other parent. In a covalent bond, the electron clouds surrounding the atomic nuclei overlap, as shown in Figure 8. Figure 8. Formation of a covalent bond between two Hydrogen atoims. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Carbon (C) is in Group IVa, meaning it has four electrons in its outer shell. Thus to become a "happy atom", Carbon can either gain or lose four electrons.

By sharing the electrons with other atoms, Carbon can become a happy atom,. alternately filling and emptying its outer shell, as with the four hydrogens shown in Figure 9. Figure 9. Formation of covalent bonds in methane. Carbon needs to share four electrons, in effect it has four slots. Each hydrogen provides an electron to each of these slots. At the same time each hydrogen needs to fill one slot, which is done by sharing an electron with the carbon. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The molecule methane (chemical formula CH4) has four covalent bonds, one between Carbon and each of the four Hydrogens. Carbon contributes an electron, and Hydrogen contributes an electron. The sharing of a single electron pair is termed a single bond. When two pairs of electrons are shared, a double bond results, as in carbon dioxide. Triple bonds are known, wherein three pairs (six electrons total) are shared as in acetylene gas or nitrogen gas. The types of covalent bonds are shown in Figure 10. Figure 10.

Ways of representing covalent bonds. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Sometimes electrons tend to spend more time with one atom in the bond than with the other. In such cases a polar covalent bond develops. Water (H2O) is an example. Since the electrons spend so much time with the oxygen (oxygen having a greater electronegativity, or electron affinity) that end of the molecule acquires a slightly negative charge. Conversely, the loss of the electrons from the hydrogen end leaves a slightly positive charge. The water molecule is thus polar, having positive and negative sides. Hydrogen bonds , as shown in Figure 11, result from the weak electrical attraction between the positive end of one molecule and the negative end of another. Individually these bonds are very weak, although taken in a large enough quantity, the result is strong enough to hold molecules together or in a three-dimensional shape. Figure 11. TOP: Formation of a hydrogen bond between the hydrogen side of one water molecule and the oxygen side of another water molecule.

BOTTOM: The presence of polar areas in the amino acids that makeup a protein allows for hydrogen bonds to form, giving the molecule a three-dimensional shape that is often vital to that protein's proper functioning. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Chemical reactions and molecules | Back to Top Molecules are compounds in which the elements are in definite, fixed ratios, as seen in Figure 12. Those atoms are held together usually by one of the three types of chemical bonds discussed above. For example: water, glucose, ATP. Mixtures are compounds with variable formulas/ratios of their components. For example: soil. Molecular formulas are an expression in the simplest whole-number terms of the composition of a substance. For example, the sugar glucose has 6 Carbons, 12 hydrogens, and 6 oxygens per repeating structural unit. The formula is written C6H12O6. Figure 12. Determination of molecular weights by addition of the weights of the atoms that make up the molecule.

Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Chemical reactions occur in nature, and some also can be performed in a laboratory setting. One such reaction is diagrammed in Figure 13. Chemical equations are linear representations of how these reactions occur. Combination reactions occur when two separate reactants are bonded together, e.g. A + B -----> AB. Disassociation reactions occur when a compound is broken into two products, e.g. AB -----> A + B. Figure 13. Diagram of a chemical reaction: the combustion of propane with oxygen, resulting in carbon dioxide, water, and energy (as heat and light). This chemical reaction takes place in a camping stove as well as in certain welding torches. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Biological systems, while unique to each species, are based on the chemical bonding properties of carbon.

Major organic chemicals (those associated with or formed by the actions of living things) usually include some ratios of the following elements: C, H, N, O, P, S. All forms of matter are composed of one or more elements. Be able to list the major elements in living things. Describe how protons, electrons, and neutrons are arranged into atoms and ions. Define the terms atomic number and atomic mass and be able to describe their sugnificance. Atoms with the same atomic number but a different mass number are isotopes. List the isotopes of hydrogen and of carbon. Be able to describe radioisotopes and list three ways they are used in biology. The union between the electron structures of atoms is known as the chemical bond. Be able to list and describe the three types of chemical bonds found in living things. Be able to describe the distribution of electrons in the space around the nucleus of an atom. An atom tends to react with other atoms when its outermost shell is only partly filled with electrons. Be able to discuss why this happens. Be able to define the two types of ions and describe thow ionic bonds form between positive and negative ions. In a covalent bond, atoms share electrons.

List several elements that tend to form covalent bonds. Distinguish between a nonpolar covalent bond and a polar covalent bond and give an example of each. Define hydrogen bond and describe conditions under which hydrogen bonds form and cite one example. Explain what is meant by the polarity of the water molecule, and how the polarity of water molecules allows them to interact with one another. Review Questions | Back to Top Which of these is not a subatomic particle? a) proton; b) ion; c) neutron; d) electron The outermost electron shell of every Noble Gas element (except Helium) has ___ electrons. a) 1; b) 2; c) 4; d) 6; e) 8 An organic molecule is likely to contain all of these elements except ___. a) C; b) H; c) O; d) Ne; e) N The chemical bond between water molecules is a ___ bond. a) ionic; b) polar covalent; c) nonpolar covalent; d) hydrogen A solution with a pH of 7 has ___ times more H ions than a solutrion of pH 9.

a) 2; b) 100; c) 1000; d) 9; e) 90 The type of chemical bond formed when electrons are shared between atoms is a ___ bond. a) ionic; b) covalent; c) hydrogen The type of chemical bond formed when oppositely charged particles are attrached to each other is a ___ bond. a) ionic; b) covalent; c) hydrogen Electrons occupy volumes of space known as ___. a) nuclei; b) periods; c) wavelengths; d) orbitals Carbon has an atomic number of 6. This means it has ___. a) six protons; b) six neutrons; c) six protons plus six neutrons; d) six neuitrons and six electrons Each of the isotopes of hydrogen has ___ proton(s). a) 3; b) 1; c) 2; d) 92; e) 1/2 A molecule is ___.

a) a mixture of various components that can vary; b) a combination of many atoms that will have different ratios; c) a combination of one or more atoms that will have a fixed ratio of its components; d) more important in a chemistry class than in a biology class Isotope | Define Isotope at Dictionary.com isotope noun, Chemistry. 1. any of two or more forms of a chemical element, having the same number of protons in the nucleus, or the same atomic number, but having different numbers of neutrons in the nucleus, or different atomic weights. There are 275 isotopes of the 81 stable elements, in addition to over 800 radioactive isotopes, and every element has known isotopic forms. Isotopes of a single element possess almost identical properties.

Origin of isotope 1910-15; iso- + -tope < Greek tópos place Related forms [ahy-suh-top-ik] /ˌaɪ səˈtɒp ɪk/ (Show IPA), adjective isotopically, adverb Examples from the Web for isotope Expand Historical Examples Why nail the "power metal" down to an isotope of gold with an atomic weight of 197? The Cosmic Computer Henry Beam Piper Well, heavy water is made of one atom of oxygen plus two atoms of deuterium, which is the first isotope of hydrogen. British Dictionary definitions for isotope Expand noun 1. one of two or more atoms with the same atomic number that contain different numbers of neutrons Derived Forms C20: from iso- + Greek topos place Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd.

1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012 Word Origin and History for isotope Expand n. 1913, literally "having the same place," introduced by British chemist Frederick Soddy (1877-1956) on suggestion of Margaret Todd, from Greek isos "equal" (see iso- ) + topos "place" (see topos ); so called because despite the different atomic weights, the various forms of an element occupy the same place on the periodic table. Online Etymology Dictionary, © 2010 Douglas Harper isotope i·so·tope (ī'sə-tōp') n. One of two or more atoms having the same atomic number but different mass numbers. i'so·top'ic (-tŏp'ĭk) adj.

The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company. isotope (ī'sə-tōp') One of two or more atoms that have the same atomic number (the same number of protons) but a different number of neutrons. Carbon 12, the most common form of carbon, has six protons and six neutrons, whereas carbon 14 has six protons and eight neutrons. Isotopes of a given element typically behave alike chemically. With the exception of hydrogen, elements found on Earth generally have the same number of protons and neutrons; heavier and lighter isotopes (with more or fewer neutrons) are often unstable and undergo radioactive decay. The American Heritage® Science Dictionary Copyright © 2002. Published by Houghton Mifflin. All rights reserved. Expand isotope [(eye-suh-tohp)] In physics , different forms of the same element , with nuclei that have the same number of protons but different numbers of neutrons .

Isotopes are distinguished from each other by giving the combined number of protons and neutrons in the nucleus. For example, uranium 235 is the isotope of uranium that has 235 protons and neutrons in its nucleus rather than the more commonly occurring 238. All elements have isotopes. The American Heritage® New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved. Slang definitions & phrases for isotope Expand isotope noun A nearly identical person; near double: Like his isotope Paglia, Rush Limbaugh can be counted on to bury the occasional nugget of truth beneath his avalanche of infuriating extrapolation and phony statistics/It actually IS you. Or an isotope of you. Or a photocopy of you [1990s+; fr the term denoting nearly identical atoms in a chemical element] The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers.

Isotopes, Isobars and Isotones | Tutorvista.com Top Isotopes, Isobars and Isotones These are the elements having same atomic number but different mass number. They have the same atomic number because the number of protons inside their nuclei remains the same. The difference in their mass number is due to the difference in their number of neutrons. Since they are neutral isotopes are elements having same number of electrons, which make them to possess identical chemical properties. Let us see some examples 1H1, 1H2, 1H3 are all isotopes of hydrogen. They all have their atomic number to be unity but the number of neutrons are 0, 1, 2 and z respectively. 17Cl37, 17Cl35 are isotopes of chlorine. They have 17 protons in the nucleus but have number of neutrons equal to 20 and 18 respectively. Practically every element consists of a mixture of several isotopes. The relative abundance of different isotopes differs from element to element.

For example chlorine is composed of two isotopes of masses 34.98U and 36.98U, which are nearly integral multiples of the mass of hydrogen atom. Their relative abundances are 75.4 and 24.6 percent respectively. Mass of natural chlorine atom can be found as = 35.47. Structure of the Atom (grades 6-8) STRUCTURE OF THE ATOM Matter has mass and takes up space. Atoms are basic building blocks of matter, and cannot be chemically subdivided by ordinary means. The word atom is derived from the Greek word atom which means indivisible. The Greeks concluded that matter could be broken down into particles to small to be seen. These particles were called atoms Atoms are composed of three type of particles: protons, neutrons, and electron. Protons and neutrons are responsible for most of the atomic mass e.g in a 150 person 149 lbs, 15 oz are protons and neutrons while only 1 oz. is electrons. The mass of an electron is very small (9.108 X 10-28 grams).

Both the protons and neutrons reside in the nucleus. Protons have a postive (+) charge, neutrons have no charge --they are neutral. Electrons reside in orbitals around the nucleus. They have a negative charge (-). It is the number of protons that determines the atomic number, e.g., H = 1. The number of protons in an element is constant (e.g., H=1, Ur=92) but neutron number may vary, so mass number (protons + neutrons) may vary. The same element may contain varying numbers of neutrons; these forms of an element are called isotopes. The chemical properties of isotopes are the same, although the physical properties of some isotopes may be different. Some isotopes are radioactive-meaning they "radiate" energy as they decay to a more stable form, perhaps another element half-life: time required for half of the atoms of an element to decay into stable form. Another example is oxygen, with atomic number of 8 can have 8, 9, or 10 neutrons. What are elements? All matter is made up of elements which are fundamental substances which cannot be broken down by chemical means.

There are 92 elements that occur naturally. The elements hydrogen, carbon, nitrogen and oxygen are the elements that make up most living organisms. Some other elements found in living organisms are: magnesium, calcium, phosphorus, sodium, potassium. By the late 1800's many elements had already been discovered. The scientist Dmitri Mendeleev, a Russian chemist, proposed an arrangement of know elements based on their atomic mass. The modern arrangement of the elements is known as the Periodic Table of Elements and is arranged according to the atomic number of elements. Here is an Interactive Table of Elements where you can learn more about each of the elements. What makes each element unique? Every atom would like to have an electron configuration like a noble gases. In noble gases the outer electron shell is complete. This makes the element chemically inert. Helium is an example of a noble (inert) gas. It is not present in organisms because it is not chemically reactive. Historical Models of the atom BOHR MODEL Bohr model shows electrons circling the nucleus at different levels or orbitals much like planets circle the sun. Electrons move from one energy state to another but can only exist aft defineite energy levels.

The energy absorbed or released when electrons change states is in the form of electromagnetic radiation. THE WAVE MODEL AND QUANTUM THEORY The Bohr model was only able to explain the very simplest atoms, like hydrogen. Today's modern day theory is based on mathematics and the properties of waves. The wave model forms the basis for the Quantum Theory . This theory gives the probability of locating electrons in a particular location, unlike assuming electrons orbit the nucleus as in the Bohr model. How are electrons organized around the nucleus? All atoms would like to attain electron configurations like noble gases. That is, have completed outer shells. Atoms can form stable electron configurations like noble gases by: sharing electrons gaining electrons. For a stable configuration each atom must fill its outer energy level. In the case of noble gases that means eight electrons in the last shell (with the exception of He which has two electrons). Atoms that have 1, 2 or 3 electrons in their outer levels will tend to lose them in interactions with atoms that have 5, 6 or 7 electrons in their outer levels. Atoms that have 5, 6 or 7 electrons in their outer levels will tend to gain electrons from atoms with 1, 2 or 3 electrons in their outer levels.

Atoms that have 4 electrons in the outer most energy level will tend neither to totally lose nor totally gain electrons during interactions. This Periodic Table of Elements will show you the electron configuration for any element you click on. Visualizing Atomic Orbitals The atomic orbitals of the hydrogen atom can be visualized as a cloud around the nucleus. The orbital represents a probability of finding the electron at a particular location. Darker regions signify a greater probability. Shown below are the 1s (lowest orbital and the 2s orbital. 1s 2s Atomic orbitals do not always have the shape of a sphere. Higher orbitals have very unusual shapes. 2px 3px These orbitals were prepared by Dr. Yue-Ling Wong from the University of Florida for more images click here. Remember molecular orbitals are 3-Dimensional Atomic Structure Atomic Structure The text provides a historical perspective of how the internal structure of the atom was discovered. It is certainly one of the most important scientific discoveries of this century, and I recommend that you read through it. However, we will begin our discussion of the atom from the modern day perspective. All atoms are made from three subatomic particles ï¿½ Protons, neutron & electrons.

These particles have the following properties: Particle 9.110 x 10-28 g 0.000549 In the above table I have used a unit of mass called the atomic mass unit (amu). This unit is much more convenient to use than grams for describing masses of atoms. It is defined so that both protons and neutrons have a mass of approximately 1 amu. Its precise definition will be given later. The important points to keep in mind are as follows: Protons and neutrons have almost the same mass, while the electron is approximately 2000 times lighter. Protons and electrons carry charges of equal magnitude, but opposite charge. Neutrons carry no charge (they are neutral). It was once thought that protons, neutrons and electrons were spread out in a rather uniform fashion to form the atom (see J.J. Thompsons plum pudding model of the atom on page 42), but now we know the actual structure of the atom to be quite different. What does an atom look like? Protons and neutrons are held together rather closely in the center of the atom.

Together they make up the nucleus, which accounts for nearly all of the mass of the atom. Electrons move rapidly around the nucleus and constitute almost the entire volume of the atom. Although quantum mechanics are necessary to explain the motion of an electron about the nucleus, we can say that the distribution of electrons about an atom is such that the atom has a spherical shape. Atoms have sizes on the order of 1-5 ï¿½ (1 angstrom = 1 ï¿½ = 1 ï¿½ 10-10 m) and masses on the order of 1-300 amu. To put the mass and dimensions of an atom into perspective consider the following analogies. If an atom were the size of Ohio stadium, the nucleus would only be the size of a small marble. However, the mass of that marble would be ~ 115 million tons. What holds an atom together? The negatively charged electron is attracted to the positively charged nucleus by a Coulombic attraction. The protons and neutrons are held together in the nucleus by the strong nuclear force. How many electrons, protons and neutrons are contained in an atom?

Atoms in their natural state have no charge, that is they are neutral. Therefore, in a neutral atom the number of protons and electrons are the same. If this condition is violated the atom has a net charge and is called an ion. The number of protons in the nucleus determines the identity of the atom. For example all carbon atoms contain six protons, all gold atoms contain 79 protons, all lead atoms contain 82 protons. Two atoms with the same number of protons, but different numbers of neutrons are called isotopes. How does the structure of the atom relate to its properties? Chemical reactions involve either the transfer or the sharing of electrons between atoms. Therefore, the chemical reactivity/ properties of an element is primarily dependent upon the number of electrons in an atom of that element. Protons also play a significant role because the tendency for an atom to either lose, gain or share electrons is dependent upon the charge of the nucleus. Therefore, we can say that the chemical reactivity of an atom is dependent upon the number of electrons and protons, and independent of the number of neutrons. The mass and radioactive properties of an atom are dependent upon the number of protons and neutrons in the nucleus.

Note: The number of protons, neutrons and electrons in an atom completely determine its properties and identity, regardless of how and where the atom was made. So it is inaccurate to speak of synthetic atoms and natural atoms. In other words a lead atom is a lead atom, end of story. It doesnt matter if was mined from the earth, produced in a nuclear reactor, or came to earth on an asteroid. Symbolism There is a symbolism or shorthand for describing atoms which is universally used across all scientific disciplines Atomic Number (Z) ï¿½ The # of protons Mass Number (A) ï¿½ [The # of protons] + [the # of neutrons] The number of protons, neutrons and electrons in an atom are uniquely specified by the following symbol ASyC Sy = The elemental symbol (i.e.

C, N, Cr) ï¿½ defines the # of protons A = The mass number ï¿½ [# of protons] + [# of neutrons] C = The net charge ï¿½ [# of protons]  [# of electrons] Example Lets start with a neutral boron 10 atom ï¿½ 10B Since the atom is a boron atom the periodic table tells us that there are 5 protons in the nucleus Z = 5. The atom is neutral so that the number of electrons must balance the number of protons, 5 electrons. The mass number is 10, so that the number of neutrons is A - Z = 10 - 5 = 5 neutrons.

Question: What are the chemically identical species of atom with the same atomic number but different mass numbers called?
Answer: Isotopes